Hirschsprung-associated enterocolitis (HAEC) is a life-threatening complication of Hirschsprung Disease (HSCR), a common cause of intestinal obstruction in the newborn that results from incomplete development of the enteric nervous system (ENS). HAEC affects 30-60% of infants with HSCR, occurs with unchanged incidence pre- and post-operatively, and carries a mortality of 5-10%, with the majority of deaths occurring in newborns prior to definitive operation. A critical barrier in the field is that the etiology of HAEC is poorly defined and current treatment remains empiric (bowel rest, rectal washouts, broad-spectrum antibiotics) and directed toward alleviating acute symptoms rather than targeting underlying pathophysiology. The long-term goal of our research is to define the pathophysiology of HAEC in order to develop novel therapeutic approaches that reduce morbidity and mortality in HSCR patients. Our preliminary and published findings, reinforced by those of other laboratories, support the central hypothesis that perturbation of host-microbiome mutualism, including evasion of immune exclusion and reinforcement of intestinal stasis by dysbiotic microbiota, drives the development of HAEC. Our objectives are to 1) define the mechanisms for impaired IgA production and secretion in HAEC, 2) identify the disease-promoting members of the dysbiotic HAEC microbiome, and 3) determine how the HAEC microbiome reinforces intestinal stasis. The proposed research is innovative because it will utilize novel, preclinical models to establish a causative relationship between dysbiosis and HAEC pathogenesis and test potential therapeutic targets. Our group is uniquely qualified to complete the aims because of our expertise in HSCR & HAEC, host- microbiome interactions, microbial endocrinology, and intestinal epithelial cell biology. The expected outcome of these studies will be a deeper understanding of the pathophysiology of HAEC and identification of novel therapeutic approaches for prevention or treatment of HAEC.